Voters could see political adverts on social media during the Senedd election campaign without knowing who is behind them.
Leaflets and print adverts must say who has produced and paid for them.
Campaigners said the Welsh government “missed an opportunity” to update election laws so the same rules applied to digital advertising for May’s vote.
The Welsh government said it wanted to work with other governments to create new regulations.
Mandatory digital imprints, which make clear who is responsible for the political adverts, will be introduced for Westminster general elections under plans outlined by the UK government.
That would mean voters could see who was trying to influence them and help watchdogs keep track of campaign spending.
Although ministers in Cardiff support the idea, it is too late for the Senedd to change the law before the election on 6 May.
Digital imprints are being made compulsory in Scotland, where devolved elections are happening on the same day.
Online advertising spending has risen sharply in British elections since 2010.
It is thought to have accounted for more than half of campaign spending at the 2019 general election and social media is predicted to be even more important now that Covid has restricted traditional canvassing.
Electoral Reform Society Cymru director Jess Blair said: “It’s a real missed opportunity that the Welsh government haven’t legislated around digital imprints for the Senedd elections this year.
“We’ve seen Scotland go ahead and do this for their elections. The legislation isn’t perfect but it’s a lot better than what we have in place in Wales.”
She said the Welsh government should not wait for UK government legislation
“You shouldn’t be waiting for the slowest person in the race,” she said.
“You should be trying to win that race and actually deliver better democracy and better transparency in our elections.”
The Welsh government said it was “committed to fair and transparent elections and digital campaigning has become an increasingly significant portion of democratic engagement from political groups”.
A spokesman said it was not possible to legislate in time for May’s election, but “we will be working closely with the other administrations of the UK to ensure there will be a robust and transparent system in the future”.
The UK government said its legislation on digital imprints would “increase transparency” and “ensure greater scrutiny”.
Cabinet Office minister Chloe Smith said it was a “complex” issue, adding: “We need to be mindful not to impose excessive regulation of free speech by individuals, nor force campaigners to publish their home addresses as part of the imprint requirement.”
The Electoral Commission previously said it was “disappointed” the rules were not changed in time for May’s Senedd election.
It has launched a campaign to help voters “think critically” about political advertising.
Spokesman Craig Westwood said: “Political adverts are an important part of how voters find out about who are standing as candidates, and what they think about key issues, but we know that people are increasingly concerned about online adverts, if they can’t easily tell who is targeting them, and why.”
Facebook said it was “constantly working to increase ad transparency and election integrity” on its platforms.
“We do not allow political ads to run without a disclaimer providing more information about who is running them,” a spokesman said.
“This includes ensuring anyone running a political ad goes through a verification process to prove who they are and that they are based in the UK, and that ads carry a ‘paid for’ disclaimer if targeting people in the UK.”
Plaid Cymru said: “Plaid Cymru will support any move to make our elections more open and transparent and digital imprints would play a key role in upholding the integrity of our elections.”
The Welsh Conservatives said the party supported digital imprints.
Welsh Liberal Democrats leader, Jane Dodds, said the party supported “transparency and fairness” in digital campaigning.
She added: “We would also, however, urge platform-owners, such as Facebook and Twitter, to make available digital imprint tools to easier facilitate this process.”